1. Field of the Invention
The present invention relates to a combination tuner capable of selectively receiving a television signal and an FM signal. More specifically, the present invention relates to a combination tuner capable of receiving a desired FM signal according to the reception level when an FM signal is received.
2. Description of the Related Art
Typically, a combination tuner is a tuner capable of selectively receiving a television signal and an FM signal, and is used in connection with a television antenna for receiving a television signal and an FM antenna for receiving an FM signal.
FIG. 3 is a circuit diagram of an exemplary known combination tuner, partially showing the main components in blocks.
As shown in FIG. 3, the combination tuner includes an external antenna 31, an internal antenna 32, a low-pass filter (LPF) 33, a band-pass filter (BPF) 34, a television signal selection stage 35, an FM signal selection stage 36, an FM trap (FM TR) 37, a radio-frequency amplifier stage (RF AMP) 38, a radio-frequency tuner stage 39, a frequency mixer stage (MIX) 40, a local oscillator (L OSC) 41, an intermediate-frequency bandwidth switching stage (IF SW) 42, an intermediate-frequency amplifier stage (IF AMP) 43, a gain setting stage 44, a switching circuit 45, an intermediate-frequency signal output terminal 46, an automatic gain control (AGC) voltage supply terminal 47, a band data supply terminal 48, and a power supply terminal 49. The external antenna 31 is a high sensitivity antenna, and is connected to the combination tuner through a connector (not shown). The internal antenna 32 is a low sensitivity antenna, and is connected to the combination tuner through a pin diode (not shown). Other circuit components are connected in such a way as shown in FIG. 3.
The television signal selection stage 35 includes three diodes 351, 352, and 353, two capacitors 354 and 355, and six resistors 356, 357, 358, 359, 3510, and 3511. These circuit components are connected in such a way as shown in FIG. 3. The FM signal selection stage 36 includes one transistor 361, three diodes 362, 363, and 364, two capacitors 365 and 366, and seven resistors 367, 368, 369, 3610, 3611, 3612, and 3613. These circuit components are connected in such a way as shown in FIG. 3. The switching circuit 45 includes one transistor 451 and one resistor 452, and these circuit components are connected in such a way as shown in FIG. 3.
The operation of the thus constructed combination tuner is now described.
A television signal received by the external antenna 31 is fed to the input of the television signal selection stage 35 after an unwanted component has been removed by the low-pass filter 33. Likewise, an FM signal received by the internal antenna 32 is fed to the input of the FM signal selection stage 36 after an unwanted component is removed by the band-pass filter 34. If the combination tuner selects to output the television signal, a ground voltage data signal is supplied to the band data supply terminal 48 from an external controller (not shown). If the combination tuner selects to output the FM signal, on the other hand, a power supply voltage data signal is supplied to the band data supply terminal 48 from the external controller.
For selection and output of the television signal, the ground voltage data signal supplied to the band data supply terminal 48 is delivered to the switching circuit 45 to turn off the transistor 451 of emitter-follower connected type so that the output of the switching circuit 45 becomes a ground voltage. The ground voltage output from the switching circuit 45 is applied to the FM signal selection stage 36, and is also applied to the control inputs of the FM trap 37, the intermediate frequency bandwidth switching stage 42, and the gain setting stage 44.
Once the FM signal selection stage 36 has received the ground voltage, the base of the transistor 361 becomes a ground voltage, and the transistor 361 is turned off. Since the transistor 361 is turned off, the diodes 362 and 363 whose cathode and anode are supplied with a positive voltage and a ground voltage, respectively, are reverse biased. The diodes 362 and 363 are then turned off. Therefore, the FM signal supplied to the input of the FM signal selection stage 36 is blocked by the diodes 362 and 363 which are turned off, and is not transmitted to the output.
In the television signal selection stage 35, since the transistor 361 is turned off, the diodes 351 and 352 whose anode and cathode are supplied with a positive voltage and a ground voltage, respectively, are forward biased, while the diode 353 whose anode and cathode are supplied with a ground voltage and a positive voltage, respectively, is reverse biased. Then, the diodes 351 and 352 are turned on, while the diode 353 is turned off. The television signal supplied to the input of the television signal selection stage 35 is therefore transmitted to the output via the diodes 351 and 352 which are turned on, followed by the FM trap 37.
Upon receipt of the ground voltage at the control input, the FM trap 37 activates an FM trap for trapping an FM frequency band signal to remove the FM signal component contained in the television signal. Upon receipt of the ground voltage at the control input, the intermediate frequency bandwidth switching stage 42 switches the intermediate frequency band to 6 MHz which is the intermediate frequency bandwidth of the television signal. Upon receipt of the ground voltage at the control input, the gain setting stage 44 sets a large signal gain for the gain setting stage 44.
The television signal output from the television signal selection stage 35, in which the FM signal component has been removed by the FM trap 37, is amplified to a predetermined level by the radio-frequency amplifier stage 38. The resulting television signal is passed to the radio-frequency tuner stage 39, where a required channel signal is selected, and is then mixed with the local oscillation signal output from the local oscillator 41 by the frequency mixer stage 40 for conversion into an intermediate frequency signal. In the mixed output from the frequency mixer stage 40, an intermediate frequency signal of 6 MHz is selected by the intermediate frequency bandwidth switching stage 42, and the resulting intermediate frequency signal is amplified to a predetermined level by the intermediate frequency amplifier stage 43. The signal is further amplified at a large signal gain by the gain setting stage 44, and is forwarded to the intermediate frequency signal output terminal 46.
For selection and output of the television signal, an automatic gain control voltage corresponding to the reception level of the television signal is further applied to the automatic gain control voltage supply terminal 47. The gain of the radio-frequency amplifier stage 38 is automatically controlled according to the automatic gain control voltage.
For selection and output of the FM signal, the power supply voltage data signal supplied to the band data supply terminal 48 is fed to the switching circuit 45 to turn on the transistor 451 of emitter-follower connected type so that the output of the switching circuit 45 becomes a power supply voltage. The power supply voltage output from the switching circuit 45 is applied to the FM signal selection stage 36, and is also applied to the control inputs of the FM trap 37, the intermediate frequency bandwidth switching stage 42, and the gain setting stage 44, as in selection and output of the television signal.
Once the FM signal selection stage 36 has received a power supply voltage, the base of the transistor 361 becomes a power supply voltage, and the transistor 361 is turned on. Since the transistor 361 is turned on, the diodes 362 and 363 whose cathode and anode are supplied with a ground voltage and a positive voltage, respectively, are forward biased. Then, the diodes 362 and 363 are turned on. At the same time, the diode 364 whose cathode and anode are supplied with positive voltage and ground voltage, respectively, is reverse biased. Therefore, the FM signal supplied to the input of the FM signal selection stage 36 is transmitted to the output via the diodes 362 and 363 which are turned on, followed by the FM trap 37.
In the television signal selection stage 35, since the transistor 361 is turned on, the diodes 351 and 352 whose anode and cathode are supplied with a ground voltage and a positive voltage, respectively, are reverse biased. Then, the diodes 351 and 352 are turned off. Therefore, the television signal supplied to the input of the television signal selection stage 35 is blocked by the diodes 351 and 352 which are turned off, and is not transmitted to the output.
Upon receipt of the power supply voltage at the control input, the FM trap 37 deactivates an FM trap for trapping an FM frequency band signal. Upon receipt of the power supply voltage at the control input, the intermediate frequency bandwidth switching stage 42 switches the intermediate frequency band to 1.2 MHz which is the intermediate frequency band of the FM signal. Upon receipt of the power supply voltage at the control input, the gain setting stage 44 sets a small signal gain for the gain setting stage 44.
For selection and output of the FM signal, the maximum gain set voltage is further applied to the automatic gain control voltage supply terminal 47 so that the gain of the radio-frequency amplifier stage 38 is controlled so as to be maximized.
The FM signal output from the FM signal selection stage 36 is not trapped by the FM trap 37, and is amplified to a predetermined level by the radio-frequency amplifier stage 38. The resulting FM signal is passed to the radio-frequency tuner stage 39, where a required frequency signal is selected, and is then mixed with the local oscillation signal output from the local oscillator 41 by the frequency mixer stage 40 for conversion into an intermediate frequency signal. In the mixed output from the frequency mixer stage 40, an intermediate frequency signal of 1.2 MHz is selected by the intermediate frequency bandwidth switching stage 42, and the resulting intermediate frequency signal is amplified to a predetermined level by the intermediate frequency amplifier stage 43. The signal is further amplified at a small signal gain by the gain setting stage 44, and is forwarded to the intermediate frequency signal output terminal 46.
As described above, the known combination tuner supplies a ground voltage data signal or a power supply voltage data signal to the band data supply terminal 48, allowing a received television signal or a received FM signal to be selected and output.
As such, the known combination tuner can select and output a received television signal or a received FM signal by changing the voltage polarity of the data signal supplied to the band data supply terminal 48. Since the known combination tuner receives an FM signal using the internal antenna 32 which is a low sensitivity antenna, the level of the received FM signal may be significantly lowered depending upon the field intensity of the FM signal received by the internal antenna 32. If an FM signal is selected and output while the level of the received FM signal is greatly lowered, the selected FM signal may not be reproduced successfully.